In alkaline pulping operations, the wood material (softwoods or hardwoods) or the lignocellulosic crop material (e.g., bagasse, kenaf, reeds, and so forth) is reacted with an alkaline cooking liquor for a given time at a specified temperature. The cooking liquor may be kraft, soda, alkaline, sulfite, polysulfide, or their modifications, such as with anthraquinone. At the desired terminating point of the delignification reaction, the cooked material still resides at high pressure and high temperature inside the digester. This is true whether the pulping process is batchwise or continuous.
Some cooling of the cooked material may then be obtained if cooler spent liquor is used to displace the hot spent liquor inside and surrounding the delignified material. This is done routinely in the countercurrent wash zones of many continuous digesters, but is uncommon in batch digester systems.
With or without prior cooling, in both batch and continuous digester operations, the delignified material is then discharged under pressure (i.e., "blown") from the digester through a pipeline and is collected in a receiving vessel which is essentially at atmospheric pressure. Hence, the pulped material is subjected to a large pressure drop during its passage from the digester to the receiving vessel. The receiving vessel is usually a blow tank in batch digester operations; it may be a blow tank or a diffusion washer in continuous digester operations.
Alkaline pulping processes, and especially kraft pulping, are the dominant means of producing cellulosic pulps worldwide. The main reason is that kraft pulping provides pulp fibers which are stronger than those from any other commercial pulping process. Even so, a serious consequence of discharging the delignified material at high pressures and high flow rates from digesters in kraft pulp mills is that the fibers experience considerable physical damage during their transfer from the pressurized pulping vessel to the unpressurized receiving vessel. This phenomenon was reported at the Canadian Pulp & Paper Association Technical Section Annual Meeting in Montreal, Quebec, Jan. 29, 1986 (J. M. MacLeod et al., Conference Proceedings, pp. A31-A37). It was shown that conventional blowing of digesters results in a substantial loss of mechanical strength of the fibers and the paper made from these fibers. This strength loss takes place during the process of blowing cooked material from both batch and continuous digesters.
There are two common means of determining the potential strength of pulp from a given pulping process. One is to conduct a well-controlled laboratory or pilot-plant cook of the lignocellulosic material under suitable chemical and process conditions. At the end of such a cook, after the pressure has been fully relieved and the temperature has fallen below 100.degree. C., the delignified material is removed manually from the small-scale digester and, under mild agitation, is disintegrated into pulp. Pulp produced in this way is the strongest possible from the lignocellulosic material which was delignified, and such pulp serves as a reference material against which the strength of a mill-made pulp can be compared.
The other means to determine the potential strength of a pulp is to use the hanging basket technique: a small, perforated basket containing the material to be pulped is suspended inside a mill digester during otherwise normal operating conditions, and the delignified material is retrieved from the basket only after all the surrounding material has been discharged from the digester. Although such basket pulps are usually slightly weaker than pilot-plant pulps, they are considerably stronger than pulps discharged by blowing from mill digesters.
Both the pilot-plant and basket procedures provide reference pulps against which the strength levels of mill-made pulps can be evaluated. Considering the superior quality of pilot-plant and basket pulps relative to conventionally-discharged mill-made pulps, it was desired to improve the latter by finding a suitable means of discharging the delignified material from mill digesters.
Although other methods of removing delignified cellulosic materials from digesters at the end of pulping have been proposed, none has been implemented in general practice in mills. For example, Gloersen and Ronnholm (Swedish Pat. No. 2407/78) propose a rotary valve arrangement which in its rotation would alternately discharge either spent liquor or delignified cellulosic material plus its entrained liquor from a batch digester. Neno (U.S. Pat. No. 4,138,311) describes a system in which an additional blow tank would be interposed between batch digesters and a conventional blow tank, the additional blow tank being operated at a pressure intermediate between the high pressure in the digester and the atmospheric pressure in the normal blow tank.
The most common means of discharging continuous digesters employs paddle-like scrapers, an outlet device, and valves and piping through which the cooked material is transferred to a receiving vessel which is maintained at atmospheric pressure. Variations on this theme have been proposed, such as in U.S. Pat. No. 3,579,421, where two discharge techniques are employed to feed separate pulp outlets at the bottom of a single continuous digester.
None of these, nor any of the common methods of discharging batch or continuous digesters, however, is able to transfer the cooked material from the digester to the blow tank or receiving vessel without imparting considerable physical damage to the cooked material being transferred, such action resulting in much weaker pulp.
In fact, only two main types of digester discharge techniques are in common use in alkaline pulp mills: (i) blowing through a valve and piping to a blow tank at atmospheric pressure in batch digester operations, and (ii) blowing through a series of depressurizing devices, valves, and piping to either a blow tank or a diffusion washer in continuous digester operations. In both of these techniques, the normal result is that the delignified fibers are damaged as they are transferred from the digesters to the receiving vessels, and hence the paper subsequently made from such fibers is inferior in its physical performance.
It is therefore the primary object of this invention to provide a method for discharging delignified cellulosic materials from digesters at the end of alkaline pulping such that the pulp fibers do not sustain severe damage, and thus can be made into paper materials which have superior strength properties.